1. Field of the Invention
The present invention pertains generally to the use of organic charge transfer salts to produce optical devices and more particularly to the use of organic charge transfer salts as a memory media for an optical memory system and as the switching mechanism for an optoelectronic switch.
2. Description of the Contemporary and/or Prior Art
With the advent of the information revolution, recent research activities have focused on developing optical storage systems and optoelectronic switches. The interaction of laser light with matter has been intensely investigated because of its potential use in optical memory systems. Potentially, optical recording can produce information storage densities in excess of 100 million bits per square centimeter. Currently optical memory devices rely on photochemical hole burning (PHB) in which a laser pits the material in an effort to store data. An article entitled "Laser Marking of a Thin Organic Film" by J. J. Wrobel et al, Applied Physics Letter 40, (11), June 1, 1982, describes such a technique using a laser beam to burn holes in a thin organic film. Similarly, optical writing on a blue, sputtered iridium oxide films is reported by Mabosch et al in Applied Physics Letter 41 (1), July, 1, 1982. This technique uses an optical writing mechanism to thermally induce dehydration at temperatures below the melting point of the optical medium. An article entitled "Light-induced Phenomena in Dye-polymer Systems" by V. Novotny et al, The Journal of Applied Physics 50 (3), March 1979, describes an optical marking process based on diffusion in a dye-polymer system.
The prior art optical storage systems have one overriding disadvantage--prior art optical media is not erasable. As a result, optical storage technology has found little application in computer technology, which requires both read, write and erase functions.
High speed solid state optoelectronic switches are currently being studied and developed for a variety of signal processing applications, including mixing, synchronous detection, analog to digital conversion and sampling. In addition, there is also considerable interest in developing integrated optoelectronic devices (IOED) for use in high speed circuits. Combined with a modulated laser, optoelectronic switches form a powerful group of integrated circuits for use in optical communications and computer technology. Such devices are currently made from highly resistive, photosensitive semiconductors which depend on the recombination of photo-generated carriers. The two basic types of semiconductor photodetectors are junction diodes and photoconductor detectors using materials such as InP and GaAs These prior art optoelectronic devices are difficult and costly to fabricate.
Two co-pending U.S. patent applications filed by R. S. Potember, T. O. Poehler and D. O. Cowan, disclose a class of organic charge transfer salts, such as CuTCNQ, which exhibits stable and reproducible switching between an equilibrium, or first state, and a second state, in the presence of an applied electrical field. These applications are: (1) "Current Controlled Bistable Electrical Organic Thin Film Switching Device (TCNQ)", filed Mar. 14, 1980, Ser. No. 130,400 now U.S. Pat. No. 4,371,883; and, (2) "Method of Fabricating a Current Control Bistable Electrical Organic Thin Film Switching Device, (TCNQ)", filed June 7, 1982, Ser. No. 385,523 now U.S. Pat. No. 4,507,672. More particularly, these applications disclose that the organic charge transfer salts will undergo a reversible electrochemical topotactic redox reaction in the presence of an applied electric field, thereby switching from a first state to a second state, and that a detectable impedance difference occurs between the equilibrium, or first state, and the second state. In specific, an electrical field is applied across a thin film of CuTCNQ, or an equivalent organic charge transfer salt. When the applied electrical field exceeds a threshold value the impedance across the thin organic film will drop from a relatively high impedance to a relatively low impedance. The application further discloses that both bistable and threshold switching are possible.
Two papers written by R. S. Potember et al report that when the organic film is electrically switched, the second state has different optical properties from the equilibrium or first state; (1) "The Vibrational and X-ray Photoelectron Spectra of Semiconducting Copper-TCNQ Films" Chemica Scripta, Vol. 17, 219-221 (1981); and (2) "Electrical Switching and Memory Phenomena in Semiconducting Organic Thin Films" American Chemical Society Symposium Series No. 184 (1982). The above articles describe infrared spectroscopic means and reference well known Raman spectroscopic techniques (S. Matsuzaki et al, "Raman Spectra of Conducting TCNQ Salts" Solid State Communications, Vol. 33, pp. 403-405, 1980) for determining if the CuTCNQ film, switched by an AC or DC electric field is in the first or second state. Follow-up work reported by E. I. Kamitsos et al (cited below) used Raman spectroscopic techniques to verify the electrochemical charge transfer equation described in the above-referenced articles which causes the CuTCNQ salt to switch from the first to second state: "Raman Study of the Mechanism of Electrical Switching in CuTCNQ films" Solid State Communications, Vol. 42, No. 8, pp. 561-565 (1982). The above cited Potember et al, Matsuzaki et al and Kamitsos et al papers point out that spectroscopic means can be used to discern whether an area of CuTCNQ switched by an applied electrical field is in the first or second state.
However, neither these papers nor the previously mentioned pending U.S. applications specifically address the use of an optical frequency beam to switch the charge transfer salts, thereby providing optoelectronic switching and an optical memory storage capability. In addition, the above-referenced papers do not specifically apply spectroscopic analysis as a means to "read" whether a part of the organic charge transfer salt is in the first or second state. Further, the above articles and applications do not disclose the use of optical switching and spectroscopic analysis as a means to optically store and retrieve information.